Electrochemical cell activated with a nonaqueous electrolyte having a sulfate additive

ABSTRACT

An alkali metal, solid cathode, non-aqueous electrochemical cell capable of delivering high current pulses, rapidly recovering its open circuit voltage and having high current capacity, is described. The stated benefits are realized by the addition of at least one organic sulfate additive to an electrolyte comprising an alkali metal salt dissolved in a mixture of a low viscosity solvent and a high permittivity solvent. A preferred solvent mixture includes propylene carbonate, 1,2-dimethoxyethane and a sulfate additive having at least one unsaturated hydrocarbon containing a C(sp or sp 2 )-C(sp 3 ) bond unit having the C(sp 3 ) carbon directly connected to the —OSO 3 — functional group.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation-in-part of application Ser.No. 09/460,035, now U.S. Pat. No. 6,180,283 to Gan et al, which is acontinuation-in-part of application Ser. No. 09/009,537, now U.S. Pat.No. 6,013,394 to Gan et al.

BACKGROUND OF INVENTION

[0002] 1. Field of the Invention

[0003] The present invention generally relates to an alkali metalelectrochemical cell, and more particularly, to an alkali metal cellsuitable for current pulse discharge applications with reduced or noappreciable voltage delay. These benefits are realized by the provisionof a sulfate additive in the electrolyte.

[0004] 2. Prior Art

[0005] The initial drop in cell potential during the application of ashort duration pulse is termed voltage delay, and reflects theresistance of the cell, i.e., the resistance due to the cathode, thecathode-electrolyte interphase, the anode, and the anode-electrolyteinterphase. In the absence of voltage delay, the resistance due topassivated films on the anode and/or cathode is negligible. However, theformation of a surface film is unavoidable for alkali metal, and inparticular, lithium metal anodes, and for lithium intercalated carbonanodes, due to their relatively low potential and high reactivitytowards organic electrolytes. Thus, the ideal anode surface film shouldbe electrically insulating and ionically conducting. While most alkalimetal, and in particular, lithium electrochemical systems meet the firstrequirement, the second requirement is difficult to achieve. In theevent of voltage delay, the resistance of these films is not negligible,and as a result, impedance builds up inside the cell due to this surfacelayer formation which often results in reduced discharge voltage andreduced cell capacity. In other words, the drop in potential between thebackground voltage and the lowest voltage under pulse dischargeconditions, excluding voltage delay, is an indication of theconductivity of the cell, i.e., the conductivity of the cathode, anode,electrolyte, and surface films, while the gradual decrease in cellpotential during the application of the pulse train is due to thepolarization of the electrodes and the electrolyte.

[0006] Thus, the existence of voltage delay is an undesirablecharacteristic of alkali metal/mixed metal oxide cells subjected tocurrent pulse discharge conditions in terms of its influence on devicessuch as medical devices including implantable pacemakers and cardiacdefibrillators. Voltage delay is undesirable because it limits theeffectiveness and even the proper functioning of both the cell and theassociated electrically powered device under current pulse dischargeconditions.

[0007] The present invention is directed to the provision of sulfateadditives in the electrolyte of an alkali metal electrochemical cell tobeneficially modify the anode surface film. The sulfate additivespreferably include at least one organic group containing a C(sp orsp²)-C(sp³) bond unit having the C(sp³) carbon directly connected to the—OSO₃— functional group, and are provided as a co-solvent with commonlyused organic aprotic solvents. The organic sulfate additives are in acondensed phase which makes them easy to handle in electrolytepreparation. When used as a co-solvent in an activating electrolyte, thesulfate additives interact with the alkali metal anode to form anionically conductive surface protective layer thereon. The conductivesurface layer improves the discharge performance of the alkali metalelectrochemical cell and minimizes or even eliminates voltage delay inthe high current pulse discharge of such cells.

SUMMARY OF THE INVENTION

[0008] The object of the present invention is to improve the pulsedischarge performance of an alkali metal electrochemical cell, and moreparticularly a primary lithium electrochemical cell, by the provision ofat least one of a family of sulfate additives as a co-solvent in thecell's activating nonaqueous electrolyte solution. Due to the highreduction potentials of the sulfate group vs. lithium, the sulfateadditives can compete effectively with the other electrolyte co-solventsor the solute to react with the lithium anode. Lithium sulfate or thelithium salt of sulfate reduction products are believed to be the majorreaction products. These lithium salts are believed to deposit on theanode surface to form an ionically conductive protective film thereon.As a consequence, the chemical composition and perhaps the morphology ofthe anode surface protective layer is changed, and this provesbeneficial to the discharge characteristics of the cell. The thuslyfabricated cell exhibits reduced or no appreciable voltage delay undercurrent pulse discharge usage, which is an unexpected result.

[0009] These and other objects of the present invention will becomeincreasingly more apparent to those skilled in the art by reference tothe following description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] As used herein, the term “pulse” means a burst of electricalcurrent of a greater amplitude than that of a pre-pulse currentimmediately prior to the pulse. A pulse train consists of at least twopulses of electrical current delivered in relatively short successionwith or without open circuit rest between the pulses. Voltage delay iscalculated as the pulse end potential minus the pulse minimum potential.

[0011] The electrochemical cell of the present invention includes ananode selected from Group IA, IIA or IIIB of the Periodic Table ofElements, including lithium, sodium, potassium, etc., and their alloysand intermetallic compounds including, for example Li—Si, Li—B andLi—Si—B alloys and intermetallic compounds. The preferred anodecomprises lithium, and the more preferred anode comprises a lithiumalloy, the preferred lithium alloy being a lithium-aluminum alloy. Thegreater the amount of aluminum present by weight in the alloy, however,the lower the energy density of the cell.

[0012] The form of the anode may vary, but preferably the anode is athin metal sheet or foil of the anode metal, pressed or rolled on ametallic anode current collector, i.e., preferably comprising nickel, toform an anode component. In the exemplary cell of the present invention,the anode component has an extended tab or lead of the same material asthe anode current collector, i.e., preferably nickel, integrally formedtherewith such as by welding and contacted by a weld to a cell case ofconductive metal in a case-negative electrical configuration.Alternatively, the anode may be formed in some other geometry, such as abobbin shape, cylinder or pellet to allow an alternate low surface celldesign.

[0013] The cathode is preferably of a solid material and theelectrochemical reaction at the cathode involves conversion of ionswhich migrate from the anode to the cathode in atomic or molecularforms. The solid cathode material may comprise a metal, a metal oxide, amixed metal oxide, a metal sulfide or a carbonaceous compound, andcombinations thereof. The metal oxide, the mixed metal oxide and themetal sulfide can be formed by the chemical addition, reaction, orotherwise intimate contact of various metal oxides, metal sulfidesand/or metal elements, preferably during thermal treatment, sol-gelformation, chemical vapor deposition or hydrothermal synthesis in mixedstates. The active materials thereby produced contain metals, oxides andsulfides of Groups IB, IIB, IIIB, IVB, VB, VIB, VIIB and VIII, whichincludes the noble metals and/or other oxide and sulfide compounds.

[0014] One preferred mixed metal oxide has the general formulaSM_(x)V₂O_(y) wherein SM is a metal selected from Groups IB to VIIB andVIII of the Periodic Table of Elements, wherein x is about 0.30 to 2.0and y is about 4.5 to 6.0 in the general formula. By way ofillustration, and in no way intended to be limiting, one exemplarycathode active material comprises silver vanadium oxide (SVO) having thegeneral formula Ag_(x)V₂O_(y) in any one of its many phases, i.e.,β-phase silver vanadium oxide having in the general formula x=0.35 andy=5.8, γ-phase silver vanadium oxide having in the general formulax=0.80 and y=5.40 and ε-phase silver vanadium oxide having in thegeneral formula x=1.0 and y=5.5, and combination and mixtures of phasesthereof. For a more detailed description of such a cathode activematerial reference is made to U.S. Pat. No. 4,310,609 to Liang et al.,which is assigned to the assignee of the present invention andincorporated herein by reference.

[0015] Another preferred composite cathode active material includesV₂O_(z) wherein z≦5 combined with Ag₂O with the silver in either thesilver(II), silver(I) or silver(0) oxidation state and CuO with thecopper in either the copper(II), copper(I) or copper(0) oxidation stateto provide the mixed metal oxide having the general formulaCu_(x)Ag_(y)V₂O_(z), (CSVO). Thus, this composite cathode activematerial may be described as a metal oxide-metal oxide-metal oxide, ametal-metal oxide-metal oxide, or a metal-metal-metal oxide and therange of material compositions found for Cu_(x)Ag_(y)V₂O_(z) ispreferably about 0.01≦x≦1.0, about 0.01≦y≦1.0 and about 5.01≦z≦ 6.5.Typical forms of CSVO are Cu_(0.16)Ag_(0.67)V₂O_(z) with z being about5.5 and Cu_(0.5)Ag_(0.5)V₂O_(z) with z being about 5.75. The oxygencontent is designated by z since the exact stoichiometric proportion ofoxygen in CSVO can vary depending on whether the cathode material isprepared in an oxidizing atmosphere such as air or oxygen, or in aninert atmosphere such as argon, nitrogen and helium. For a more detaileddescription of this cathode active material reference is made to U.S.Pat. Nos. 5,472,810 to Takeuchi et al. and 5,516,340 to Takeuchi et al.,both of which are assigned to the assignee of the present invention andincorporated herein by reference.

[0016] Additional cathode active materials include manganese dioxide,lithium cobalt oxide, lithium nickel oxide, copper vanadium oxide,vanadium oxide, titanium disulfide, copper oxide, copper sulfide, ironsulfide, iron disulfide, carbon, fluorinated carbon, and mixturesthereof. Preferably, the cathode comprises from about 80 to about 99weight percent of the cathode active material.

[0017] Cathode active materials prepared as described above arepreferably mixed with a binder material such as a powderedfluoro-polymer, more preferably powdered polytetrafluoroethylene orpowdered polyvinylidene fluoride present at about 1 to about 5 weightpercent of the cathode mixture. Further, up to about 10 weight percentof a conductive diluent is preferably added to the cathode mixture toimprove conductivity. Suitable materials for this purpose includeacetylene black, carbon black and/or graphite or a metallic powder suchas powdered nickel, aluminum, titanium and stainless steel. Thepreferred cathode active mixture thus includes a powdered fluoro-polymerbinder present at about 3 weight percent, a conductive diluent presentat about 3 weight percent and about 94 weight percent of the cathodeactive material. The cathode active mixture may be in the form of one ormore plates operatively associated with at least one or more plates ofanode material, or in the form of a strip wound with a correspondingstrip of anode material in a structure similar to a “jellyroll”.

[0018] In order to prevent internal short circuit conditions, thecathode is separated from the Group IA, IIA or IIIB anode material by asuitable separator material. The separator is of electrically insulativematerial, and the separator material also is chemically unreactive withthe anode and cathode active materials and both chemically unreactivewith and insoluble in the electrolyte. In addition, the separatormaterial has a degree of porosity sufficient to allow flow therethroughof the electrolyte during the electrochemical reaction of the cell.Illustrative separator materials include woven and non-woven fabrics ofpolyolefinic fibers or fluoropolymeric fibers including polyvinylidenefluoride, polyethylenetetrafluoroethylene, andpolyethylenechlorotrifluoroethylene laminated or superposed with apolyolefinic or a fluoropolymeric microporous film. Suitable microporousfilms include a polytetrafluoroethylene membrane commercially availableunder the designation ZITEX (Chemplast Inc.), polypropylene membranecommercially available under the designation CELGARD (Celanese PlasticCompany, Inc.) and a membrane commercially available under thedesignation DEXIGLAS (C. H. Dexter, Div., Dexter Corp.). The separatormay also be composed of non-woven glass, glass fiber materials andceramic materials.

[0019] The form of the separator typically is a sheet which is placedbetween the anode and cathode electrodes and in a manner preventingphysical contact therebetween. Such is the case when the anode is foldedin a serpentine-like structure with a plurality of cathode platesdisposed intermediate the anode folds and received in a cell casing orwhen the electrode combination is rolled or otherwise formed into acylindrical “jellyroll” configuration.

[0020] The electrochemical cell of the present invention furtherincludes a nonaqueous, ionically conductive electrolyte operativelyassociated with the anode and the cathode electrodes. The electrolyteserves as a medium for migration of ions between the anode and thecathode during the electrochemical reactions of the cell and nonaqueoussolvents suitable for the present invention are chosen so as to exhibitthose physical properties necessary for ionic transport (low viscosity,low surface tension and wettability). Suitable nonaqueous solvents arecomprised of an inorganic salt dissolved in a nonaqueous solvent andmore preferably an alkali metal salt dissolved in a mixture of aproticorganic solvents comprising a low viscosity solvent including organicesters, ethers, dialkyl carbonates, and mixtures thereof, and a highpermittivity solvent including cyclic carbonates, cyclic esters, cyclicamides, and mixtures thereof. Low viscosity solvents includetetrahydrofuran (THF), methyl acetate (MA), diglyme, triglyme,tetraglyme, 1,2-dimethoxyethane (DME), dimethyl carbonate (DMC),diisopropylether, 1,2-diethoxyethane, 1-ethoxy,2-methoxyethane, diethylcarbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC),methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), andmixtures thereof. High permittivity solvents include propylene carbonate(PC), ethylene carbonate (EC), butylene carbonate (BC), acetonitrile,dimethyl sulfoxide, dimethyl formamide, dimethyl acetamide,γ-valerolactone, γ-butyrolactone (GBL), N-methyl-pyrrolidinone (NMP),and mixtures thereof.

[0021] The preferred electrolyte comprises an inorganic alkali metalsalt, and in the case of an anode comprising lithium, the alkali metalsalt of the electrolyte is a lithium based salt. Known lithium saltsthat are useful as a vehicle for transport of alkali metal ions from theanode to the cathode include LiPF₆, LiBF₄, LiAsF₆, LiSbF₆, LiClO₄,LiAlCl₄, LiGaCl₄, LiC(SO₂CF₃)₃, LiO₂, LiN(SO₂CF₃)₂, LiSCN, LiO₃SCF₂CF₃,LiC₆F₅SO₃, LiO₂CCF₃, LiSO₃F, LiB(C₆H₅)₄, LiCF₃SO₃, and mixtures thereof.Suitable salt concentrations typically range between about 0.8 to 1.5molar, and a preferred electrolyte for a lithium/transition metal oxideelectrochemical cell includes LiAsF₆ or LiPF6 dissolved in a 50:50mixture, by volume, of PC and DME.

[0022] A distinguishing characteristic of a primary lithium metal cell,in comparison to that of a secondary lithium ion cell, is that thelithium anode material is consumed during discharge. In other words, thesurface of the anode is shrinking or fading away from theelectrode/electrolyte interphase during discharge. This is especiallythe case during high current pulse discharge when the anode surfacepassivation film breaks up to expose a fresh lithium surface. At thisstage, a new passivation film forms and its chemical composition andsurface morphology depends on the competition reactions of the existingcomponents in the electrolyte, which, in turn, depends on the cell depthof discharge.

[0023] In a lithium/silver vanadium oxide cell, there are twocharacteristic voltage plateaus in the discharge profile. The first oneoccurs at about 3.2V and the second plateau occurs at about 2.6V. At the3.2V discharge plateau, Li/SVO cells are known to be relatively stableand exhibit consistent long term pulse discharge behavior. Thisindicates that the lithium anode is effectively passivated by theelectrolyte. However, when the cell is discharged to the 2.6V plateau,voltage delay is present in long term pulse discharge tests.

[0024] It has been determined that voltage delay in an alkalimetal/transition metal oxide cell is caused by the growth of anodesurface impedance. The chemical composition and the passivation abilityof the anode surface layer change in the time that the Li/SVO celldischarges from the 3.2V plateau to the 2.6V plateau. Consequently, theanode surface impedance increases over time and results in voltage delayduring high current pulse discharge in long term tests.

[0025] In the absence of a sulfate additive to the electrolyte accordingto the present invention, the anode surface passivation film, whichreforms on the newly exposed, fresh lithium surface during discharge,becomes more resistive ionically, or does not effectively passivate theanode electrically, and therefore, additional amounts of electrolytedecomposition occur on the anode surface. In turn, the impedance of theanode passivation layer increases over long term discharge tests, whichresults in unacceptable voltage delay and decreased pulse minimumpotentials over those experienced without the undesirable anode surfacepassivation film. This is the reason for voltage delay during highcurrent pulse discharge, especially in long term tests when the cell hasbeen depleted of 40% to 70% of its capacity.

[0026] According to the present invention, the undesirable or “bad”passivation film formed on the lithium anode at the 2.6V depth ofdischarge in a Li/SVO cell is prevented by the presence of anelectrically insulating and ionically conductive or “good” passivationlayer formed on the anode surface. This passivation layer does not breakup, or does not break up easily, during high current pulse discharge. Ifthe passivation film does break up, it is readily reformed before anundesirable passivation layer replaces it. Thus, it is believed thatexposure of fresh lithium is prevented or minimized during discharge.

[0027] An electrically insulating and ionically conducting or “good”passivation layer is formed on the lithium anode in accordance with thepresent invention by the provision of at least one sulfate additive as aco-solvent in the electrolyte solution of the previously describedalkali metal electrochemical cell. The sulfate additive preferably hasthe general formula R¹OS(═O)₂(OR²), wherein at least one of R¹ and R² isan organic group containing at least 3 carbon atoms and having an sp orsp² hydridized carbon atom bonded to an sp³ hydridized carbon atombonded to the oxygen atom bonded to the sulfur atom, or those were atleast one of R¹ and R² is an unsaturated inorganic group. Sulfateadditive coming under the purview of the present invention include thosewhere at least one of R¹ and R² has a first bond structure of the typeC(sp)-C(sp³) directly connected to the —OSO₃— functional group, thosewhere at least one of R¹ and R² has a second bond structure of the typeC(sp²)-C(sp³) directly connected to the —OSO₃— functional group, orthose where at least one of R¹ and R²is an unsaturated inorganic group.Sulfate compounds having at least one of the first or the second bondstructures or which have an unsaturated inorganic group comprise, forexample, R¹ and R² selected from the group of benzyl, allyl, propargyland cyanomethyl. In that case, the other of R¹ and R² not being thefirst bond structure or the second bond structure is either a linear ora cyclic alkyl group having 1 to 13 carbon atoms or an unsaturatedinorganic group. Exemplary alkyl groups include: methyl, ethyl, propyl,iso-propyl, butyl, iso-butyl, tert-butyl, pentyl, hexly, cyclopropyl,cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl,cyclononanyl, cyclodecanyl, cycloundecyl, cyclododecyl, phenyl, tolyland cyanomethyl.

[0028] Examples of sulfate compounds having at least one of R¹ and R² ofthe first bond structure of the type C(sp)-C(sp³) directly connected tothe —OSO₃— functional group include:

[0029] dipropargyl sulfate (R=propargyl) or

[0030] “alkyl” propargyl sulfate (R≠propargyl)

[0031] Examples of sulfate compounds having at least one of R¹ and R²being an unsaturated inorganic group include:

[0032] dicyanomethyl sulfate (R=cyanomethyl) or

[0033] cyanomethyl “alkyl” sulfate (R≠cyanomethyl)

[0034] Examples of sulfate compounds having at least one of R¹ and R² ofthe second bond structure of the type C(sp²)-C(sp³) directly connectedto the —OSO₃— functional group include:

[0035] dibenzyl sulfate (R=benzyl) or

[0036] “alkyl” benzyl sulfate (R≠benzyl)

[0037] diallyl sulfate (R=allyl) or

[0038] “alkyl” allyl sulfate (R≠allyl)

[0039] The above described compounds are only intended to be exemplaryof those that are useful with the present invention, and are not to beconstrued as limiting. Those skilled in the art will readily recognizesulfate compounds which come under the purview of the general formulaset forth above and which will be useful as additives for theelectrolyte to reduce voltage delay according to the present invention.

[0040] While not intending to be bound by any particular mechanism, itis believed that due to the presence of the sulfate additive comprisingat least one unsaturated hydrocarbon containing a C(sp or sp²)-C(sp³)bond unit having the C(sp³) carbon directly connected to the —OSO₃—functional group or being an unsaturated inorganic group, the bondbetween oxygen and at least one of the R¹ and R² groups is readilysevered and the sulfate intermediate is able to compete effectively withthe other electrolyte solvents or solutes to react with lithium and forma sulfate salt of the formula S(═O₂) (O—Li) _(n)(OR)_(m) where n=1 or 2and m=0 or 1, i.e., lithium sulfate, or the lithium salt of a sulfatereduction product on the surface of the anode. The resulting salt ismore conductive than lithium oxide which may form on the anode in theabsence of the organic sulfate additive. In fact, it is believed thatthe lithium sulfate or the lithium salt of a sulfate reduction producton the surface of the anode provides for the existence of chargedelocalization due to resonance equilibration at the anode surface. Thisequilibration allows lithium ions to travel easily from one molecule tothe other via a lithium ion exchange mechanism. As a result, beneficialionic conductance is realized.

[0041] In the present invention, the anode is lithium metal and thecathode is preferably the transition mixed metal oxide AgV₂O_(5.5)(SVO). The preferred electrolyte is 1.0M to 1.2M LiAsF₆ dissolved in anaprotic solvent mixture comprising at least one of the above listed lowviscosity solvents and at least one of the above listed highpermittivity solvents. The preferred aprotic solvent mixture comprises a50/50, by volume, mixture of propylene carbonate and1,2-dimethoxyethane. The concentration of the above discussed sulfateadditives according to the present invention should preferably be in therange of between about 0.001M to about 0.40M. The positive effects ofthese additives in reducing voltage delay in a pulse discharging alkalimetal cell have been achieved both at room temperature as well as attemperatures up to about 37° C. This makes the novel electrolytesolution of the present invention particularly useful for activating analkali metal/transition metal oxide cell incorporated into animplantable medical device such as a cardiac defibrillator to minimizeor even eliminate voltage delay under high current pulse dischargeconditions.

[0042] As is well known by those skilled in the art, an implantablecardiac defibrillator is a device that requires a power source for agenerally medium rate, constant resistance load component provided bycircuits performing such functions as, for example, the heart sensingand pacing functions. From time to time, the cardiac defibrillator mayrequire a generally high rate, pulse discharge load component thatoccurs, for example, during charging of a capacitor in the defibrillatorfor the purpose of delivering an electrical shock to the heart to treattachyarrhythmias, the irregular, rapid heartbeats that can be fatal ifleft uncorrected. Reduction and even elimination of voltage delay duringa current pulse application is important for proper device operation andextended device life.

[0043] The assembly of the cell described herein is preferably in theform of a wound element cell. That is, the fabricated cathode, anode andseparator are wound together in a “jellyroll” type configuration or“wound element cell stack” such that the anode is on the outside of theroll to make electrical contact with the cell case in a case-negativeconfiguration. Using suitable top and bottom insulators, the wound cellstack is inserted into a metallic case of a suitable size dimension. Themetallic case may comprise materials such as stainless steel, mildsteel, nickel-plated mild steel, titanium, tantalum or aluminum, but notlimited thereto, so long as the metallic material is compatible for usewith components of the cell.

[0044] The cell header comprises a metallic disc-shaped body with afirst hole to accommodate a glass-to-metal seal/terminal pin feedthroughand a second hole for electrolyte filling. The glass used is of acorrosion resistant type having up to about 50% by weight silicon suchas CABAL 12, TA 23 or FUSITE 425 or FUSITE 435. The positive terminalpin feedthrough preferably comprises titanium although molybdenum,aluminum, nickel alloy, or stainless steel can also be used. The cellheader comprises elements having compatibility with the other componentsof the electrochemical cell and is resistant to corrosion. The cathodelead is welded to the positive terminal pin in the glass-to-metal sealand the header is welded to the case containing the electrode stack. Thecell is thereafter filled with the electrolyte solution comprising atleast one of the sulfate additives described hereinabove andhermetically sealed such as by close-welding a stainless steel ball overthe fill hole, but not limited thereto.

[0045] The above assembly describes a case-negative cell, which is thepreferred construction of the exemplary cell of the present invention.As is well known to those skilled in the art, the exemplaryelectrochemical system of the present invention can also be constructedin a case-positive configuration.

[0046] It is appreciated that various modifications to the inventiveconcepts described herein may be apparent to those of ordinary skill inthe art without departing from the spirit and scope of the presentinvention as defined by the appended claims.

What is claimed is:
 1. An electrochemical cell, which comprises: a) ananode comprising an alkali metal; b) a cathode comprising a cathodeactive material; c) a non-aqueous electrolyte operatively associatedwith the anode and the cathode, the non-aqueous electrolyte comprising asulfate additive having the general formula R¹OS (═O₂) (OR²), wherein R¹and R² are selected from the group consisting of: i) at least one of R¹and R² is an organic group containing at least 3 carbon atoms and havingeither a first bond structure of the type C(sp)-C(sp³) directlyconnected to the —OSO₃— functional group or having a second bondstructure of the type C(sp²)-C(sp³) directly connected to the —OSO₃—functional group; and ii) at least one of R¹ and R² is an unsaturatedinorganic group.
 2. The electrochemical cell of claim 1 wherein when thesulfate additive has at least one of R¹ and R² having the first bondstructure, the second bond structure, or being the unsaturated inorganicgroup, then the at least one of R¹ and R² is selected from the groupconsisting of benzyl, allyl, propargyl and cyanomethyl.
 3. Theelectrochemical cell of claim 2 wherein the other of R¹ and R² which isnot the first bond structure, the second bond structure, or theunsaturated inorganic group is either a linear or a cyclic organic grouphaving 1 to 13 carbon atoms or an inorganic group.
 4. Theelectrochemical cell of claim 1 wherein the sulfate additive is presentin the electrolyte in a range of about 0.001M to about 0.40M.
 5. Theelectrochemical cell of claim 1 including discharging the cell todeliver at least one current pulse of an electrical current of a greateramplitude than that of a pre-pulse current immediately prior to thepulse.
 6. The electrochemical cell of claim 5 wherein there are at leasttwo pulses delivered in succession with or without an open circuitperiod between the pulses.
 7. The electrochemical cell of claim 1wherein the nonaqueous electrolyte comprises a first solvent selectedfrom the group consisting of an ester, an ether, a dialkyl carbonate,and mixtures thereof, and a second solvent selected from the groupconsisting of a cyclic carbonate, a cyclic ester, a cyclic amide, andmixtures thereof.
 8. The electrochemical cell of claim 7 wherein thefirst solvent is selected from the group consisting of tetrahydrofuran,methyl acetate, diglyme, triglyme, tetraglyme, 1,2-dimethoxyethane,diisopropylether, 1,2-diethoxyeltane, 1-ethoxy,2-methoxyethane, dimethylcarbonate, diethyl carbonate, dipropyl carbonate, ethyl methylcarbonate, methyl propyl carbonate, ethyl propyl carbonate, and mixturesthereof.
 9. The electrochemical cell of claim 7 wherein the secondsolvent is selected from the group consisting of propylene carbonate,ethylene carbonate, butylene carbonate, acetonitrile, dimethylsulfoxide, dimethyl formamide, dimethyl acetamide, γ-valerolactone,γ-butyrolactone, N-methyl-pyrrolidinone, and mixtures thereof.
 10. Theelectrochemical cell of claim 1 including providing an alkali metal saltin the electrolyte, the alkali metal salt being selected from the groupconsisting of LiPF₆, LiBF₄, LiAsF₆, LiSbF₆, LiClO₄, LiAlCl₄, LiGaCl₄,LiC(SO₂CF₃)₃, LiO₂, LiN(SO₂CF₃)₂, LiSCN, LiO₃SCF₂CF₃, LiC₆F₅SO₃,LiO₂CCF₃, LiSO₃F, LiB(C₆H₅)₄, LiF₃SO₃, and mixtures thereof.
 11. Theelectrochemical cell of claim 1 wherein the cathode active material isselected from the group consisting of silver vanadium oxide, coppersilver vanadium oxide, vanadium oxide, copper vanadium oxide, andmixtures thereof.
 12. The electrochemical cell of claim 1 wherein theanode is comprised of lithium or a lithium-aluminum alloy.
 13. Theelectrochemical cell of claim 1 wherein the cathode comprises from about80 to about 99 weight percent of the cathode active material.
 14. Theelectrochemical cell of claim 1 wherein the cathode further comprises abinder material and a conductive additive.
 15. The electrochemical cellof claim 14 wherein the binder material is a fluoro-resin powder. 16.The electrochemical cell of claim 14 wherein the conductive additive isselected from the group consisting of carbon, graphite powder, acetyleneblack, titanium powder, aluminum powder, nickel powder, stainless steelpowder, and mixtures thereof.
 17. The electrochemical cell of claim 1wherein the cathode comprises from about 0 to 3 weight percent carbon,about 1 to 5 weight percent of a powder fluoro-resin and about 94 weightpercent of the cathode active material.
 18. The electrochemical cell ofclaim 1 including powering an implantable medical device with theelectrochemical cell.
 19. A method for reducing voltage delay in a pulsedischarging electrochemical cell activated with a nonaqueouselectrolyte, comprising the steps of: a) providing an anode comprisingan alkali metal; b) providing a cathode including a mixed metal oxidecomprised of vanadium oxide and a second metal “SM” selected from thegroup consisting of Groups IB, IIB, IIIB, IVB, VIB, VIIB and VIII of thePeriodic Table of the Elements, the mixed metal oxide having the generalformula SM_(x)V₂O_(y), wherein 0.30≦x≦2.0 and 4.5≦y≦6.0; c) activatingthe electrochemical cell with the non-aqueous electrolyte operativelyassociated with the anode and the cathode, the non-aqueous electrolytecomprising: i) a first solvent selected from the group consisting of anester, an ether, a dialkyl carbonate, and mixtures thereof; ii) a secondsolvent selected from the group consisting of a cyclic carbonate, acyclic ester, a cyclic amide, and mixtures thereof; iii) a sulfateadditive having the general formula R¹OS(═O₂) (OR²), wherein at leastone of R¹ and R is an organic group containing at least 3 carbon atomsand being selected from the group consisting of a first bond structureof the type C(sp)-C(sp³) directly connected to the —OSO₃— functionalgroup, a second bond structure of the type C(sp²)-C(sp³)- directlyconnected to the —OSO₃— functional group, and an unsaturated inorganicgroup; and iv) an alkali metal salt dissolved therein; and d)discharging the cell to deliver at least one current pulse of anelectrical current of a greater amplitude than that of a pre-pulsecurrent immediately prior to the pulse.
 20. The method of claim 19including providing the sulfate additive having at least one of R¹ andR² having the first bond structure, the second bond structure, or beingthe unsaturated inorganic group, then the at least one of R¹ and R² isselected from the group consisting of benzyl, allyl, propargyl andcyanomethyl.
 21. The method of claim 20 including providing the other ofR¹ and R² which is not of the first bond structure, the second bondstructure, or the unsaturated inorganic group is either a linear or acyclic organic group having 1 to 13 carbon atoms or an inorganic group.22. The method of claim 19 wherein the sulfate additive is present inthe electrolyte in a range of about 0.001M to about 0.40M.
 23. A methodfor providing an electrochemical cell, comprising the steps of: a)providing an anode comprising an alkali metal; b) providing a cathode ofa cathode active material; c) activating the anode and the cathode witha non-aqueous electrolyte comprising a sulfate additive having thegeneral formula R¹OS (═O²) (OR²), wherein at least one of the R¹ and Ris an organic group containing at least 3 carbon atoms and beingselected from the group consisting of a first bond structure of the typeC(sp)-C(sp³) directly connected to the —OSO₃— functional group, a secondbond structure of the type C(sp²)-C(sp³) directly connected to the—OSO₃— functional group, and an unsaturated inorganic group.
 24. Themethod of claim 23 including providing the sulfate additive having atleast one of R¹ and R² having the first bond structure, the second bondstructure, or being the unsaturated inorganic group, then the at leastone of R¹ and R² is selected from the group consisting of benzyl, allyl,propargyl and cyanomethyl.
 25. The method of claim 24 wherein the otherof R¹ and R² which is not the first bond structure, the second bondstructure, or the unsaturated inorganic group is either a linear or acyclic organic group having 1 to 13 carbon atoms or an inorganic group.26. The method of claim 23 including providing the electrolytecomprising a first solvent selected from the group consisting of anester, an ether, a dialkyl carbonate, and mixtures thereof, and a secondsolvent selected from the group consisting of a cyclic carbonate, acyclic ester, a cyclic amide, and mixtures thereof.
 27. The method ofclaim 23 including providing the electrolyte having an alkali metal saltdissolved therein.
 28. The method of claim 23 wherein the sulfateadditive is present in the electrolyte in a range of about 0.001M toabout 0.40M.
 29. The method of claim 24 wherein in the cathode activematerial is selected from the group consisting of silver vanadium oxide,copper silver vanadium oxide, manganese dioxide, lithium cobalt oxide,lithium nickel oxide, copper vanadium oxide, vanadium oxide, titaniumdisulfide, copper oxide, copper sulfide, iron sulfide, iron disulfide,carbon, fluorinated carbon, and mixtures thereof.